High power triggering switches have broad application and are used in conjunction with radar microwave source switching, particle accelerators, gyrotrons, free electron lasers, and relativistic magnetrons. In particular, such high power switches are used to trigger excimer and other gas lasers.
Excimer lasers require an active medium, such as an excited rare-gas (i.e. ArF, KrF, or XeCl). In order to achieve this acceleration of electrons, high voltage and amperage is necessary.
Likewise, doped-insulator lasers, such as ruby optical medium lasers, may be rapidly "pumped" optically by a xenon flashlamp which operates with current densities in excess of 100 amperes/cm.sup.2. High voltage (in excess of 20 KV) pulses are needed to ionize the gas inside the xenon flashlamp. The flashlamp must operate rapidly, providing a current path for discharging high amperage current which flows through the flashlamp.
In addition to the ability to carry high voltage and high current (resulting in a megawatt power transfer), switches used in conjunction with lasers and optical pumps, like those switches that activate the xenon flashlamp, must be operated with a fast risetime in order to create a population inversion in the active lasing medium.
Due to the need for any switch used to trigger and pulse a laser optical pump to be able to carry a high voltage, high current, and high current density, as well as being able to rapidly switch yet control voltage breakdown with a fast rise time, semiconductor switches are not the best suited pulse generators for high power laser system applications.
Among the electronic switches used as a trigger or pulse generator in a laser system is the thyratron, a gas-filled triode tube having an anode, cathode, and control grid. The thyratron operates in either a quiescent state where no current is passed or in a firing state which allows the controlled discharge of high current through its ionized gas chamber. One type of thyratron is the model HY3202 manufactured by EG & G, Inc., Electro-Optics Division, Salem, Mass. (for use in conjunction with the Lambda Physik Laser), which is a heated cathode design. Unlike conventional thyratrons which are triggered at their grid, the model HY3202 thyratron receives a trigger pulse at its cathode, which renders a grounded-grid positive with respect to the cathode. Electrons injected from the heated cathode, after triggering, travel through the grid space and into the region between a positive anode and the grounded grid. A low impedance spark arises between the anode and grid, allowing high current to flow in the tube while protecting the cathode from excess wear. (An improved version of the Model HY 3202 is manufactured by English Electric Valve, Ltd. of Chelmford, Essex, England.) Thyratrons offer fast risetime, low internal resistance, easy ignition by an external trigger and rapid recovery; but, they suffer from electrode wear.
The pseudospark, a high powered fast switch, was disclosed by D. Bloess, et al., in Nuclear Instruments Methods 205, 173 (1983). In this article the authors describe a multigap "pseudospark" chamber for producing a controlled trigger mechanism for the fast switch. The thyratron and "pseudospark" chamber switches, generally operate according to a law of electro-physics known as Paschen's law. Paschen's law defines the ability of gases to hold off a large voltage before "breakdown" and current flow as a function of the product gas pressure and the spacing between electrodes.
Experimental plots verifying Paschen's law, when plotted on a graph (See FIG. 5) having the horizonal axis measured in (p.d) (the product of pressure times distance between anode and cathode or cathode and grid) against a vertical axis measured in breakdown voltage, looks like a U-shaped curve (in (p.d) ranges above vacuum conditions). At values above 25 to 40 mbar.mm, a near linear rise appears, so that high pressure tubes having electrodes positioned a fixed distance apart show greater control the farther electrodes are positioned from one another. This region is known as the right side (62) of the Paschen curve (An example of an electronic switch operating in this right side will be discussed later in this section).
The Paschen curve bottoms out at about 7-13 mbar.mm, and as pressures are lowered below this range down to 10.sup.-3 Torr, (a Torr equals approximately 1 mbar; 760 Torr equals one atmosphere pressure) the left side (56) of the Paschen curve exhibits a sharp rise in the curve. On this left side of the Paschen curve, by precisely and sharply lowering internal gas pressure within a gas filled tube, control of triggering may be maintained into the high voltage range. Grid-controlled thyratrons and the triggered pseudospark chamber described by Bloess operate on the left side of the Paschen curve in the lower distance-pressure regions. These switches, however, due to the need for a physical grid, have a higher impedance; and, the grid and cathode (even in cold cathode switches) experience degradation and limited life. Also, triggering by means of a physical grid interposed between the anode and cathode means use an electrode trigger which is electrically coupled to the controlled high powered circuit. This electrical coupling of the controlled main circuit (necessarily high power to trigger lasers) to the trigger necessarily introduces inherent safety problems.
To avoid the safety problems of a grid which is coupled to the controlled anode-to-cathode circuit, a laser triggered spark gap was proposed in the November, 1965 issue of the Review of Scientific Instruments, Volume 36, at page 1546, authored by A. H. Guenther, et al.
The spark gap switch proposed by Guenther et al. comprised a high pressure gas-filled chamber having two 5 cm diameter stainless steel anode and cathode electrodes spaced 1.5 cm apart. A focused laser beam was directed by a 50 mm lens at a focal point midway between the two electrodes along a line joining their respective centers. Controlled voltage breakdown and flow of current between the electrodes was initiated by the focused laser beam. Gas pressure in the chamber of this spark gas switch was held at about 600 Torr (or nearly one atmosphere gas pressure). The laser focal point was moved over a range between 0.335 cm and 0.0 cm from the cathode. Focusing the laser beam on the surface of the cathode electrode indicated arc formation by thermionic emission of electrons from the electrode surface, as well as blow off of gaseous products from a surface irradiated by a focused laser. These ejected gases may be composed of electrode material and absorbed gaseous dielectric. Guenther reported that the largest triggerable region (outside the self-breakdown region (64) of the Paschen curve) occurs when the focused laser beam is directed onto the surface of the cathode.
A laser triggered spark gap of the prior art is shown in FIG. 6. A focused laser beam 72 is directed at the surface of the cathode 76. It will be noted that cathode 76 and anode 78 are separated by a wide gap between the electrodes in the region of 1.5 cm. The cathode and anode of the Guenther apparatus are preferably spherical and are placed in a high pressured gas filled container 80. Triggering of this switch is initiated when the high powered laser beam 72, focused on the surface of the cathode 76, creates a plasma from the cathode material which lifts into the gas filled chamber and ionizes the gas between the electrodes causing a spark gap across the electrodes to form.
Guenther concluded (at page 1550) that a laser triggered high pressure spark gap switch has low jitter, short response time, and good switching technique, due to the electrical isolation of the optically coupled focused laser beam trigger. He also concludes and teaches that high pressure triggering is easier and safer than lower pressure triggering.
Thus, the state of the art clearly indicates that laser triggered spark gap switches operate at high pressures with a relatively wide gap between adjacent spherical electrodes. The use of a high power focused laser beam to trigger the switch (on the right side of the Paschen curve) results in the laser or subsequent streamer arcing producing permanent cathode damage. At the other end of the Paschen curve, the left side, thyratrons and "pseudospark" switch operate but are triggered by electrically coupled grids providing impedance and delay in rise time, as well as control grid and anode degradation.
It is an object of this invention to provide a high power electronic switch which draws from the advantages of the designs of grid controlled thyratrons, as well as laser triggered spark gap with as good or better statistical parameters during operation of the switch and which will provide an electronic switch having good operating characteristics over a longer life.
Such a switch should perform in a variety of environments, including that of a high acceleration particle system.
An electronic switch is needed with improved standoff voltage, peak current, current rate of rise, lifetime, and low jitter repetition rate.
These and other objects are addressed by the disclosed invention herein.